Lignite has a total carbon content of about 25-35%, a moisture content that is sometimes as high as about 66%, an ash content of about 6-19%, and a heat content ranging from about 10-20 MJ/kg (i.e. about 9-17 million Btu per short ton) on a dry ash-free (DAF) basis. For comparison, bituminous coal has a total carbon content of about 60-80%, an ash content of about 6-12%, and a calorific value of about 24-35 MJ/kg on a DAF basis. Sub-bituminous coals have total carbon contents, ash contents, and heating values between lignite and bituminous coals, and contain on the order of 15-30% moisture. It should be noted that all of these content ranges are exemplary only, and non-limiting (as used throughout this Specification). Lignite has a high content of volatile matter, which makes it easier to convert into gas and liquid petroleum products than other higher-ranking coals. However, lignite's high moisture content and susceptibility to spontaneous combustion may cause difficulties in transportation and storage.
Because of its low energy density, lignite is inefficient to transport and is not traded extensively in world markets as compared to other higher-ranking coals. Lignite is often used for fuel in power stations that are constructed close to a coal mine. Australia's Latrobe Valley and Luminant's Monticello plant in Texas are examples of lignite-burning power plants that are located close to a coal mine.
Lignite does not have enough strength to be used in a blast furnace as coke, even if the lignite is devolatilized. Lignite has less fixed carbon, which makes it difficult to use in direct reduction (DR) and other iron-making processes as the carbonaceous material that is agglomerated with an iron oxide containing material.
Sub-bituminous coal is similar to lignite and has a typical moisture content of about 20-30%. The high moisture content of the sub-bituminous coal also causes its energy density to be lower than other higher-ranking coals. Further, sub-bituminous coal does not have enough strength to be used in a blast furnace as coke, even if it is devolatilized.
Thus, conventional methods and systems for producing DRI and/or hot metal typically use coke or high-grade coal, and lignite (i.e. brown coal), sub-bituminous coal, and the like are not used. Lignite is used only for local, low-efficiency power generation. The low efficiency of this power generation results from the fact that part of the energy of the coal must be used to vaporize the moisture in order to dry the lignite and sub-bituminous coal.